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Full text of "Text Book Of Mechanical Engineering"

542

Stresses in Wire Rope.

:;

<<;:

pit-head; and A is thereby caught, being slipped relatively down-
ward. The jaws then open or catch on B, as at D.

Fig. 554 shews Fowler's travelling crane driven by wire rope
round clip pullies. A is the rope arrangement, and the power is
distributed for travelling at c c, cross traversing at r>, and lifting at
E F. The last is accomplished by the rotation of screw F, which
shortens the lifting chain attached to nut E. The arrangement is
suitable for very heavy cranes.

Cable tramways are useful for bad inclines. An endless rope
travels in a conduit A, Fig. 555, and the car carries the gripping
lever B, which, when moved to the vertical, raises the rollers c c,
and brings the jaws D D together. Some jerk is, of course,
unavoidable.

Fig. 557 is a towing arrangement adopted on some German
canals. A rope is anchored on the canal bottom, and the
tug winds itself along by the engine-driven clip pulley. The
rope serves as a rail, and with the pulley forms a kinematic
pair.

In wire-rope transmission the tension ratio is usually 2 : i and
the speed 3000 to 6000 feet per m. The stresses in the rope are
due to:

(1)  Weight of rope and the form of hanging curve.

(2)  Bending of rope round pulley.

(3)  Centrifugal force.

(i) In Fig. 558 the catenaries may be considered as parabolas
for all practical purposes. Then the tangent T A being drawn, by
bisecting c D at A, the force diagram will give the value of T, in
terms of W the weight of rope between the pullies, and B the
pressure on the bearing. The weight of wire rope per foot =
(1*34 x d*} Ibs. (2) Taking the general bending formulae,

El      J     ^
Bm==_==/      /Z

p      y

and I = Zy
where p = radius of pulley, andj' that of the rope-wire:

= 30,000,000 .